Conventionally, active energy ray-curable inkjet inks, which can be dried more quickly than solvent-type inkjet inks, have been often used in signage printers for high speed printing. Such active energy ray-curable inkjet ink also has high adhesion to base materials, and thus has been often used in flat bed printers for printing on a variety of base materials. Further, the ink compositions have been developed to meet applications.
These printers have a scanning head to meet an increase in size, in coating thickness, or in density. In recent years, as print head technology has advanced, technology that enables a printing head to eject micro-droplets at high frequency has been established. With the merit of digitization, the achievement of this technology has increased the chance of replacing traditional printing methods with inkjet printing, which used to be inferior in productivity or image quality. However, this technological innovation requires the development of ink compositions having high-frequency suitability better than that of conventional ones and having low viscosity and high sensitivity. Lower ink viscosity contributes to an improvement in the accuracy of droplet landing, and thus is a feature required of ink for achieving high-definition image quality. In addition, the substitution of inkjet printing for traditional offset printing also requires an improvement in color reproducibility. Unfortunately, it has been very difficult, particularly for an active energy ray-curable inkjet ink, to satisfy all required properties including image color reproducibility, curing properties, and discharge stability.
If the amount of the application of ink composition droplets is increased to achieve wide color reproducibility, cracking of the cured coating may occur to disturb the image or to degrade the fixity. If an ink composition is prepared with a higher concentration of a pigment so that wide color reproducibility can be obtained, the ink composition may have higher viscosity or contain a higher concentration of coarse particles. This may cause a problem with discharge stability over a long time.
Literature 1 discloses a set of color inkjet ink for use in printing, which not only include magenta, yellow, and cyan ink commonly used, but also include an orange ink containing a water-soluble orange dye and/or a green ink containing a green dye and/or a violet ink containing a violet dye. This enables the production of an image having a better color gamut than that obtained using a traditional ink set. Literature 2 discloses an ink with good color reproducibility and high productivity, which is produced by using a combination of a specific sensitizer and at least one organic pigment having an orange, violet, or green color. However, these techniques can increase the cost and size of the equipment because an increase in the number of ink colors means an increase in the number of printing heads. Thus, it is practically difficult to unlimitedly increase the number of ink colors, and thus there has been a limit to the available image quality.
Literatures 3 and 4 disclose a study in which the color gamut of offset printing is widened using a pigment containing a metal lake pigment of a rhodamine dye. In this study, however, large amounts of resin and a hexafunctional monomer are used, so that the resulting compositions have very high viscosity. Thus, these ink compositions are very difficult to discharge from existing inkjet heads, and it has been desired to create an ink composition suitable for inkjet printing.